initial ascent phase. Hydraulic fluid for operating these actuators is stored in an externally mounted cylindrical tank at the base of each booster. These boosters burn for 130 seconds and produce an average thrust of and a peak thrust of each. The second stage, designated L110, is a liquid-fueled stage that is tall and wide, and contains of unsymmetrical dimethylhydrazine (UDMH) and nitrogen tetroxide (). It is powered by two Vikas 2 engines, each generating thrust, giving a total thrust of . The L110 is the first Indian clustered liquid-fueled engine. The Vikas engines uses regenerative cooling, providing improved weight and specific impulse compared to earlier Indian rockets. Each Vikas engine can be individually gimbaled to control vehicle pitch, yaw and roll control. The L110 core stage ignites 114 seconds after liftoff and burns for 203 seconds. Since the L110 stage is air-lit, its engines need shielding during flight from the exhaust of the operating S200 boosters and reverse flow of gases by a 'nozzle closure system' which gets jettisoned prior to L110 ignition. The cryogenic upper stage, designated C25, is in diameter and long, and contains of propellant LOX and LH2, pressurized by helium stored in submerged bottles. It is powered by a single CE-20 engine, producing of thrust. CE-20 is the first cryogenic engine developed by India which uses a gas generator, as compared to the staged combustion engines used in GSLV. The CFRP composite payload fairing has a diameter of and a payload volume of . Variants and upgrades Mating with semi-cryogenic stage The L110 core stage in the GSLV Mk III is planned to be replaced by the SC120, a kerolox stage powered by the SCE-200 engine to increase its payload capacity to to geostationary transfer orbit (GTO). The SCE-200 uses kerosene instead of unsymmetrical dimethylhydrazine (UDMH) as fuel and has a thrust of around 200 tonnes. Four such engines can be clustered in a rocket without strap on boosters to deliver up to to GTO. The first propellant tank for the SC120 was delivered in October 2021 by HAL. The SC120-powered version of GSLV will not be used for the crewed mission of the Gaganyaan spacecraft. In September 2019, in an interview by AstrotalkUK, S. Somanath, director of Vikram Sarabhai Space Centre claimed that the SCE-200 engine was ready to begin testing. As per an agreement between India and Ukraine signed in 2005, Ukraine was